1. Field of the Invention
The present invention relates to a relatively compact image pickup device to be loaded in a camera phone or a like device; and a driving device, to be incorporated in the image pickup device, for driving a lens unit constituting an image pickup optical system for focus control or zoom control.
2. Description of the Related Art
In recent years, a high-resolution image pickup device has been developed. For instance, the number of pixels of an image sensor to be loaded in a camera phone or a like device has been spectacularly increased. As the high-resolution device has been developed, a high-performance lens unit constituting an image pickup optical system has also been demanded. Specifically, a fixed focal length system is replaced by an auto-focus system. Concerning a zoom function, an optical zoom function has been demanded in place of or in addition to a digital zoom function. In view of the above trend, an actuator for moving a lens element in an optical axis direction is required both in the auto-focus system and the optical zoom system.
As an actuator, there is known a shape memory alloy (hereinafter, also called as “SMA”) actuator. The SMA actuator is constructed in such a manner that a contraction force is generated by energizing and heating an SMA member, and the contraction force is used as a lens driving force. Generally, the SMA actuator is advantageous in easily reducing the size and the weight of a lens driver, and obtaining a relatively large magnitude of force.
FIG. 1 is a diagram schematically showing an arrangement example of a lens unit 10. The lens unit 10 is constructed in such a manner that a lens barrel 2 for holding a lens element 1 is slidably movable along a guide rod 5 provided between fixing members 3 and 4. A bias spring 6 is mounted between the lens barrel 2 and the fixing member 3. A SMA member 7 is mounted between the lens barrel 2 and the fixing member 4. Both terminals of the SMA member 7 are connected to a driving circuit 8. In the case where a drive current is applied from the driving circuit 8 to the SMA member 7, or a currently applied drive current is increased to heat the SMA member 7, the SMA member 7 is contracted, thereby holding the lens barrel 2 as a driven member i.e. the lens element 1 at a position where the contraction force of the SMA member 7 is balanced with a resilient force of the bias spring 6. On the other hand, in the case where application of the drive current to the SMA member 7 is suspended, or a currently applied drive current is decreased to cool the SMA member 7, the SMA member 7 is expanded, and the lens barrel 2 i.e. the lens element 1 is held at a position balanced with the resilient force of the bias spring 6.
In the lens unit 10 having the above arrangement, the displaced position of the lens barrel 2 i.e. the lens element 1 can be detected by measuring a resistance value of the SMA member 7. FIG. 2 is a graph showing a relation between a displacement amount of an SMA member, and a resistance value of the SMA member. As shown in FIG. 2, there is known that a resistance value of an SMA member is monotonously changed in a certain range of a displacement amount to a maximum displacement amount Xmax corresponding to a resistance value Rmin, depending on the length of the SMA member. For instance, Japanese Unexamined Patent Publication No. 2006-183564 (D1) discloses an arrangement, wherein a constant voltage is applied to a series circuit constituted of an SMA member and a resistor, and a resistance value of the SMA member is obtained based on a divided voltage thereof; or a Wheatstone bridge circuit is formed by parallel-connecting series circuits constituted of two resistors as a comparator, and a resistance value of the SMA member is obtained based on a difference between divided voltages.
In the conventional art, a resistance value of the SMA member is obtained by applying a constant voltage to the series circuit constituted of the SMA member and the resistor, and detecting a voltage at a connected point between the SMA member and the resistor. In the above arrangement, a resistor having a relatively high precision is required. In the case where an integrated circuit is fabricated, the SMA member and the resistor are externally mounted. This may increase the production cost and the size of the SMA actuator, and cause a large electric power loss by the resistor.